1. Field of the Invention
The present invention relates to an interferometer system and interferometer system processing method whereby information position of an object under measurement is detected by detecting light reflected by direct measurement light having a high coherency onto the object under measurement, and also to a method for signal processing and a stage that measures the position of a moving object using a signal processing method.
2. Description of Related Art
An interferometer system is used to measure the position information of an object under measurement by directing a measurement light having high coherency, such as a laser beam or light, onto the object to be measured and detecting the reflected light, and has a high resolution such as 1 nm. For this reason, a laser interferometer is provided in an exposure apparatus that transfers a pattern onto a mask for manufacture of semiconductor element liquid-crystal elements, magnetic heads, or other microdevices, or it is provided transfers a pattern onto a glass plate, and is used to measure the position information of a stage which causes the mask or substrate to move.
An interferometer system has a light source that emits high-coherency light, and divides the high-coherency light emitted by the light source into a plurality of parts, one thereof being used as the above-noted measurement light. For example, in an interferometer system provided in an exposure apparatus, the high-coherency light from a light source is divided into three, these being used as the above-noted measurement light, a reference light which is directed onto a fixed mirror that establishes a reference position with respect to the above-noted stage, and a reference light for establishing a reference light path that has a known light path link for the purpose of obtaining position information of the stage.
The stage position information is measured by directing measurement light onto a moving mirror provided on the stage and causing interference between the obtained reflected light and the referenced light reflected from the fixed mirror, the interference light being detected by a receiver and a comparison being made between the detection signal obtained from the receiver and the reference signal obtained by detecting the reference light using a reference receiver via the reference light path. The detection signal of the receiver for the purpose of measuring the position information and the reference signal of the reference receiver are both digitized (converted to binary form), and when noise is superimposed on either the detection signal or the reference signal, there is a sudden change in the period of the signal (a so-called glitch), which causes a reduction in the measurement accuracy of the position information. In order to prevent such problems, there has been disclosed, in Japanese Unexamined Patent Application, First Publication No. 2001-94401, a glitch-reduction circuit that reduces glitches when a detection signal is digitized.
The above-noted glitch-reduction circuit reduces the glitch occurring upon digitization of the detection signal or reference detection signal, but does not completely eliminate the glitch at the point in time of the digitization. For this reason, the phenomenon of a sudden variation of the pulse width of a digital signal that is converted and the phenomenon of a sudden loss of the pulse for one or more periods still occurs. When these phenomena occur, an error occurs leading to the problem of a loss in accuracy of measurement of the position information.
In recent years, there has arise a demand for reduced design rules in order to increase the density of device manufacturing. In particular in the manufacture of semiconductor elements, the process rule is tending to be reduced to approximately 0.1 μm, and there is an extreme degree of precision that is required in overlapping patterns to be exposed onto a substrate with patterns that have already been formed thereon. In order to improve the precision of overlaying, it is first necessary to measure the position information of the stage with high accuracy, and for this reason, it is necessary to avoid measurement errors that are caused by noise as much as possible.